It all depends on where your mail server is. If it's on a publicly accessable computer (with a public ip address), then it's just a case of installing the right software and you can then use imap/pop3/webmail/whatever.
However, I'm assuming your server is internal to your network. This is what we do, it may not be the best way - but it works.
Our gateway computer (windows 2000 server) with a connection to the internet has Terminal Services and vpn setup. We also have outlook installed on this computer, and have given the senior partners vpn usernames and passwords.
They can then connect to this computer using vpn, and then run a terminal services connection to it which will let them not just check their emails - but also run the specialist software our company uses.
Again, there's probably a better way for just email access - but that's what we do. Hope this helps in some way.